1 Technical Field of the Invention
The present invention relates generally to an FM-CW radar apparatus which may be employed in anti-collision systems or cruise control systems installed in moving objects such as automotive vehicles and which is designed to transmit a frequency-modulated radar wave and receive a return of the radar wave from a target object to determine the distance to and relative speed of the target object.
2 Background Art
Recently, radars are used in automotive vehicles to measure the distance to and relative speed of an object present ahead of the vehicle. As one of such radars, an FM-CW (frequency modulated-continuous wave) radar is now proposed which is designed to transmit a radar wave which is frequency-modulated with a triangular wave to have a frequency increasing and decreasing cyclically, receive a radar return of the transmitted radar wave from a target, and mix the received radar wave with the transmitted one to produce a beat signal. The frequency of the beat signal (referred to as a beat frequency below) is determined using a signal processor in each of ranges wherein the frequency of the transmitted radar wave increases and decreases. The frequency of the beat signal in the range wherein the frequency of the transmitted radar wave increase will be referred to as a rising beat frequency, and that range will be referred to as a modulated frequency rising range. Similarly, the frequency of the beat signal in the range wherein the frequency of the transmitted radar wave decreases will be referred to as a falling beat frequency, and that range will be referred to as a modulated frequency falling range. If the rising beat frequency is defined as fb1, and the falling beat frequency is defined as fb2, the distance D to and relative speed V of a target may be expressed by the equations (1) and (2) below. EQU D=(C/(8.multidot..DELTA.F fm)).multidot.(fb1+fb2) (1) EQU V=(C/(4.multidot.f0)).multidot.(fb2-fb1) (2)
where .DELTA.F is a variation in frequency of the transmitted radar wave, f0 is the central frequency of the transmitted radar wave, 1/fm is the time required for modulation in one cycle (i.e., fm is the frequency of the triangular wave used in modulating the transmitted radar wave), and C is the velocity of light.
FIGS. 4(a) and 4(b) show frequency relations between a signal T transmitted from the FM-CW radar and a signal R received by the FM-CW radar.
FIG. 4(a) illustrates for the case where a moving object equipped with the FM-CW radar and a target are identical in speed with each other, that is, where the relative speed V of the moving object to the target is zero (0). Usually, a return of a radar wave from a target undergoes a delay of time the radar wave takes to travel from the radar to the target and back. Thus, the received signal R is, as shown in the drawing, shifted in phase from the transmitted signal T along a time axis so that the rising beat frequency fb1 will be, as shown in FIG. 4(a), equal in peak level to the falling beat frequency fb2.
FIG. 4(b) illustrates for the case where a moving object equipped with the FM-CW radar and a target are different in speed from each other, that is, where the relative speed V of the moving object to the target is not zero. In this case, the received signal R is further doppler-shifted in frequency as a function of the relative speed V so that the received signal R is shifted in frequency from the transmitted signal T, which will cause, as shown in FIG. 4(d), the rising beat frequency fb1 to be different in peak level from the falling beat frequency fb2.
The determination of the distance D to and relative speed V of the target may, thus, be accomplished with use of the above relations between the rising beat frequency fb1 and the falling beat frequency fb2.
However, in practical use, the radar usually detects a plurality of targets and produces as many peak frequencies as the targets in each of the modulated frequency-rising and -falling ranges. Particularly, when the speed of the radar is different from those of the targets, the rising beat frequency fb1 and the falling beat frequency fb2 show, as described above, different peak levels because of the doppler shift. Therefore, when a plurality of peak frequencies are detected, as described above, in each of the modulated frequency-rising and -falling ranges, pairing each of the peak frequencies in the modulated frequency-rising range and one of the peak frequencies in the modulated frequency-falling range in a simple manner will result in difficulty in determining the distance D to and the relative speed V of each target accurately.
U.S. Pat. No. 5,619,208 issued on Apr. 8, 1997, assigned to the same assignee of this application teaches an FM-CW radar system designed to avoid the above problem. This system combines each of peak frequencies in the modulated frequency-rising range with one of peak frequencies in the modulated frequency-falling range to produce a peak frequency pair and determines an estimate thereof which may be expected to appear a given time later. If the estimated peak frequency pair is close to a corresponding pair of peak frequencies which have appeared in the modulated frequency-rising and -falling ranges actually the given time later, the system determines the estimated peak frequency pair as a correct peak frequency pair and derive data on the target using it.
The above system, however, has the drawback in that the selection of the estimated peak frequency pairs as the correct peak frequencies consumes much time, thus resulting in delay in outputting data on each target and is unuseful for the case where there is a moving object such as a vehicle nipping in front of a system-equipped vehicle suddenly which needs to be identified quickly.